U.S. patent number 4,764,219 [Application Number 06/923,245] was granted by the patent office on 1988-08-16 for clean up and passivation of mercury in gas liquefaction plants.
This patent grant is currently assigned to Mobil Oil Corporation. Invention is credited to Tsoung Y. Yan.
United States Patent |
4,764,219 |
Yan |
August 16, 1988 |
Clean up and passivation of mercury in gas liquefaction plants
Abstract
Gas liquefaction apparatus contaminated with mercury is
decontaminated by circulating through the apparatus a solvent
containing sulfur or a gas containing a sulfur reactant such as
sulfur, hydrogen sulfide or alkyl thiol.
Inventors: |
Yan; Tsoung Y. (Philadelphia,
PA) |
Assignee: |
Mobil Oil Corporation (New
York, NY)
|
Family
ID: |
25448374 |
Appl.
No.: |
06/923,245 |
Filed: |
October 27, 1986 |
Current U.S.
Class: |
134/2;
148/DIG.17; 422/7; 422/9; 423/210 |
Current CPC
Class: |
A62D
3/33 (20130101); B01J 19/002 (20130101); C09K
3/32 (20130101); A62D 2101/43 (20130101); Y10S
148/017 (20130101) |
Current International
Class: |
A62D
3/00 (20060101); B01J 19/00 (20060101); C09K
3/32 (20060101); B01D 053/34 (); C09K 003/00 () |
Field of
Search: |
;423/210,561B ;55/72
;502/516 ;422/7,9 ;148/DIG.17 ;106/14.45 ;165/5
;252/8.3,8.552,8.555,80,87,89.1 ;134/2 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
72199 |
|
Oct 1975 |
|
AU |
|
84722 |
|
May 1982 |
|
JP |
|
20224 |
|
Feb 1983 |
|
JP |
|
833287 |
|
May 1981 |
|
SU |
|
Other References
The Analytical Chemistry of Sulfur and its Compounds, Kartchmer,
ed. Wiley-Interscience, 1970, pp. 465, 467..
|
Primary Examiner: Doll; John
Assistant Examiner: Russel; Jeffrey Edwin
Attorney, Agent or Firm: McKillop; Alexander J. Gilman;
Michael G. Harrison, Jr.; Van D.
Claims
What is claimed is:
1. A process for passivating mercury present as a contaminant on
gas processing equipment comprising contacting said mercury with a
liquid solvent containing at least in part sulfur or a
sulfur-containing compound selected from the group consisting of
free elemental sulfur, colloidal sulfur, hydrogen sulfide,
monosulfides, polysulfides and alkyl thiols said liquid solvent
being selected from the group consisting of benzene, toluene,
methanol, butane, propane, gas condensate, ethane, ethanol,
propanol, carbon disulfide and mixtures thereof.
2. The process of claim 1 wherein the sulfur or sulfur-containing
compound is free elemental sulfur or colloidal sulfur.
3. The process of claim 1 wherein the sulfur-containing compound is
hydrogen sulfide.
4. The process of claim 1 wherein the sulfur-containing compound is
an alkyl thiol of the structure RSH where R is an alkyl group of 1
to 10 carbon atoms.
5. The process of claim 1 wherein the sulfur containing compound is
selected from the group consisting of sodium polysulfide and
ammonium sulfide and mixtures of the two.
6. The process of claim 1 wherein the process is conducted at a
temperature between about -10.degree. C. and -70.degree. C.
7. A process for passivating mercury present as a contaminant on
gas processing equipment comprising contacting said mercury with a
liquid solution containing, at least in part, sulfur or a
sulfur-containing compound selected from the group consisting of
free elemental sulfur, colloidal sulfur, hydrogen sulfide,
monosulfides, polysulfides and alkyl thiols and a liquid solvent
selected from the group consisting of benzene, toluene, methanol,
butane, propane, gas condensate, ethane, ethanol, propanol, carbon
disulfide and mixtures thereof.
8. The process of claim 7 wherein the sulfur or sulfur-containg
compound is free elemental sulfur or colloidal sulfur.
9. The process of claim 7 wherein the sulfur-containing compound is
hydrogen sulfide.
10. The process of claim 7 wherein the sulfur-containing compound
is an alkyl thiol of the structure RSH where R is an alkyl group of
1 to 10 carbon atoms.
11. The process of claim 7 wherein the sulfur-containing compound
is selected from the group consisting of sodium polysulfide and
ammonium sulfide and mixtures of the two.
12. The process of claim 7 wherein the temperature is between about
-10.degree. C. and -70.degree. C.
Description
NATURE OF THE INVENTION
This invention relates to the purification of natural gas. More
specifically, this invention relates to a method for removing and
cleaning gas liquefaction apparatus which has been contaminated
with mercury.
BACKGROUND OF THE INVENTION
Raw natural gas must be treated prior to its liquefaction for
several reasons. These include removing compounds which interfere
with the liquefaction process, with the separation and recovery of
hydrocarbon liquids and with meeting the specifications set for the
recovered products. For example, the gas must be dried to prevent
ice formation during cryogenic operations. Hydrogen sulfide
ordinarily must be removed because of its toxic nature. A large
number of commercial processes are in use for treating and
separating of raw wellhead gas. The steps used in these different
processes are each well known to those skilled in the art.
Some natural gas contains mercury at levels as high as 200 to 300
micrograms per cubic meter. For example, the mercury level of
natural gas produced at one field in Indonesia is about 250
micrograms per cubic meter. Concentrations of mercury at this level
creates safety hazards and air pollution problems. Refinery
equipment such as heat exchangers can be adversely effected by the
action of accumulated mercury. The problem of mercury in natural
gas is discussed further in U.S. Pat. No. 4,094,777 and French Pat.
No. 2,310,795, both of which are incorporated herein by
reference.
Crude natural gas containing mercury ordinarily is treated by first
flowing it through a bed containing sulfur distributed over a
carbon support. The free sulfur present reacts with mercury in the
natural gas and removes it from the natural gas. The gas is then
contacted with an alkali carbonate to remove the carbon dioxide and
hydrogen sulfide present in the gas and subsequently is treated by
liquid amine extraction to remove any residual hydrogen sulfide.
The gas is then dehydrated to remove water and finally is cooled
and liquefied after treatment in a heat exchanger. It is the heat
exchange equipment which is a primary source of problems resulting
from mercury contamination. Ordinarily the heat exchangers are made
of aluminum which is easily corroded and ultimately destroyed by
the cumulative effect of mercury present in the natural gas.
Although the mercury can be removed by contact with the
sulfur-on-carbon absorbent, the mercury content can be lowered only
to a level of from 250 to 0.03 micrograms per cubic meter. This
lower level is considered to be the minimum concentration
achievable under the prevailing thermodynamic limitations. As the
mercury removing system ages, however, the mercury level in the
effluent gas will increase up to 0.1 micrograms per cubic meter or
higher over a number of years. The mercury content thus may reach
levels which are considered too high for the continued safe
operation of the aluminum heat exchangers. This is because the
mercury tends to condense on the cold surfaces of the heat
exchanger and there to react with the aluminum leading to its
ultimate corrosion and failure.
Furthermore operating experience has shown that the
mercury-removing equipment upstream (e.g. sulfur-on-carbon
absorbent beds) sporadically malfunctions. Consequently mercury in
the natural gas is not removed but is carried through the gas
system to a point where it contacts aluminum equipment such as the
heat exchangers. This malfunction of the upstream mercury removal
equipment has been found to contribute significantly to the overall
mercury corrosion problem.
Even though the mercury does not immediately react with the
aluminum it may still tend to accumulate on the surface of the
aluminum and where the aluminum is not protected by a shield of
aluminum oxide or if the aluminum oxide coating becomes scratched,
the mercury will further react with the aluminum.
A primary purpose of this invention therefore is to provide a
method of passivating, or rendering non-reactive, residual mercury
present in gas liquefaction equipment. Still another object of this
invention is to prevent the further corrosion and deterioration of
gas liquefaction equipment that has become contaminated by the
accumulation of mercury.
SUMMARY OF THE INVENTION
Briefly stated, this invention comprises passivating, or rendering
non-reactive mercury present in equipment to render the mercury
harmless by contacting the equipment with a flowing stream of gas
containing a sulfur-containing reactant or a liquid solution
containing sulfur or a sulfur compound either when the equipment is
in use or when use of the equipment has been suspended for purposes
of repair and renewal.
DESCRIPTION OF THE INVENTION
Applicant's application can be utilized in at least two aspects,
one directed to the situation where the particular gas treating
facility has been shut down (unit turnaround) and the other to the
situation where the gas continues to be processed and it is
desirable to effect mercury passivation during the gas processing
phase.
In a system where all activity toward liquefying the natural gas
has been suspended and the system is undergoing renovation and
repair, the mercury on the contaminated apparatus can be removed by
circulating through it a solvent such as benzene, toluene,
methanol, butane, propane, gas condensate, ethane, ethanol,
propanol, carbon disulfide and mixtures thereof. Preferably the
circulation of solvent through the apparatus is conducted at a
temperature between -40.degree. C. and -70.degree. C. The solvent,
after it has passed through the mercury contaminated equipment and
has removed at least some of the mercury, is carried to a solvent
purification system where the mercury content of the solvent is
reduced or removed. Removal can be effected by contacting the
solvent with sulfur, copper, bismuth, zinc, gold or silver
dispersed on a carbon support or by running the solvent through a
column packed with silver gauze, and by filtration. The solvent is
then recycled back through the apparatus to remove more mercury. It
has been determined that between 0.1 and 1 grams of mercury can be
suspended in a gallon of solvent under these conditions. The
solvent effectively removes the mercury from every part of the
system where it circulates. Circulation of the solvent is continued
until the mercury content of the solvent is reduced to less than
1.times.10.sup.-4 grams per gallon.
The above-described process can be accelerated by incorporating
into the cleaning solvent dissolved sulfur, sulfides, polysulfides
and colloidal sulfur in a concentration between 0.001 and 1 percent
by weight and circulating the resulting mixture. The polysulfides
are particularly preferred. Polysulfides which are most preferred
are sodium, ammonium, and other forms which are soluble in the
solvent selected for circulation in the system. For application in
aluminum equipment, the pH of the resulting mixture can be lowered
to below 9 by adding mineral or organic acids. Preferably the pH is
maintained between 7 and 7.5. The sulfur species react with the
mercury so that more total mercury can be dissolved or suspended in
the solvent to accelerate the cleaning operation. Because
polysulfides are very reactive, passivation can be conducted at low
temperature for example, below -40.degree. C. to alleviate mercury
corrosion during the passivation operation. The effluent liquid
solution is removed from the system and treated to remove mercury
and mercuric sulfide such as by passing it over a column of sulfur
dispersed on a carbon support or by filtration. The solvent can
then be recycled.
As indicated previously, it is also possible to conduct this
invention during the actual treatment of the gas. This can be
effected in one of two ways.
In the first method a passivating agent, such as free sulfur,
hydrogen sulfide or alkyl thiol of the formula RSH where R is a
hydrocarbon chain of 1 to 10 carbon atoms, or mixtures thereof, are
injected intermittently after the dehydrator and upstream of the
heat exchanger. The concentration of the passivating agent in the
injected stream can be between about 1 parts per bilion to about 10
ppm. Injection of the passivating agent is suspended when its
presence in the amount of about 10 percent begins to appear in the
natural gas product flowing to the LNG equipment. These passivating
agents react, of course, with the mercury present in the apparatus
and render it inactive. Much of the agent is also deposited in the
system at locations where mercury will likely deposit subsequently.
This contributes to the effectiveness of this aspect of the
invention.
If gas processing can be suspended while the equipment is being
treated for mercury contamination, a reactive sulfur gas such as
hydrogen sulfide, alkyl thiols, free sulfur or other gas can be
circulated through the equipment until a suitable degree of
passivation is attained.
In another aspect of the invention a scrubber is positioned
immediately after the dehydrator unit to remove heavy hydrocarbons
such as butanes and pentanes plus. This scrubber solvent is
saturated with free sulfur which reacts with mercury present in the
gases being scrubbed to form small amounts of harmless mercuric
sulfide which are separated out by sedimentation or filtration. The
temperature in the scrubber is sufficiently low, for example,
0.degree. F., so that the thermodynamic limitations of the reaction
for mercury removal are greatly improved.
Of the procedures discussed above the method utilizing anhydrous
hydrogen sulfide gas in the liquid or gas phase is considered to be
the best mode and preferred method. At a temperature of -10.degree.
C. the mercury is sufficiently liquid to facilitate reaction with
the hydrogen sulfide but it will not harm the aluminum surfaces it
contacts.
At a time when it is proposed to passivate the mercury in a section
of equipment dry hydrogen sulfide gas is introduced into the
section either in gaseous form or in solution in a liquid
hydrocarbon such as liquid butane. The section is maintained at a
pressure of 0 to 100 psig and a temperature of -10.degree. C. for a
period of one to two days to complete the passivation reaction. The
same conditions are used if other sulfur-containing materials are
used in lieu of hydrogen sulfide. When the passivation reaction is
completed, the temperature of the section is increased further to
volatilize any other condensed liquids in the section. In a final
step the system is purged of hydrogen sulfide, mercuric sulfide and
other debris by flushing with a natural gas or liquid, such as
liquid butane.
Use of anhydrous hydrogen sulfide gas is believed to be important.
Moisture carried in with the gas could aggravate mercury corrosion.
Dry hydrogen sulfide is also preferred because of its reactivity
and relative simplicity in handling.
EXAMPLES
I
Deposition of Mercury from Vapor onto Aluminum
The deposition of mercury from the LNG in an aluminum heat exhanger
was simulated in the following procedure. An aluminum coupon of
11/4.times.11/4.times.1/16 inches with a hole of 1/8-inch diameter
at the center was thoroughly polished with steel wool. The coupon
was placed horizontally in a Soxhlet extractor without the thimble.
In the flask of the extractor 250 cc of toluene and 5 grams of
mercury were heated to boiling (110.degree. C.). The toluene vapor
containing the mercury was condensed and the condensate flowed over
the coupon. This refluxing operation was continued for about 20
hours. The mercury condensed along with the toluene was deposited
on the coupon. The coupon with the mercury deposit was examined
with a microscope. Examination showed the definite presence of
mercury globules on the aluminum surface.
II
Passivation of Mercury
The demonstration set-up is shown in FIG. 1. Mercury (10 grams) was
charged to a glass tube fitted with a medium glass frit of 1 cm
diameter. An aluminum coupon was hung in the space to show any
effects of dry hydrogen sulfide gas on aluminum. Hydrogen sulfide
gas was introduced from the bottom and bubbled slowly through the
mercury for 6 hours. The excess hydrogen sulfide gas was scrubbed
with caustic solution. The remaining gas was hydrogen produced in
the reaction between hydrogen sulfide and mercury. By measuring the
rate of hydrogen gas evolved, one can calculate the reaction rate
between mercury and hydrogen sulfide. In the interim the aluminum
coupon was visually inspected and showed no visible reaction with
the dry hydrogen sulfide.
A similar test was conducted in which the mercury was immersed in
n-hexane. The result showed that the reaction between mercury and
the dissolved hydrogen sulfide was considerably slower than that
with dry hydrogen sulfide gas.
III
Reactivity of Mercury in Cold Solvent
A mixture of solvents (methanol, 90% and acetone, 10%) was chilled
to -60.degree. C. The methanol contained 200 ppm of sodium
polysulfide. Mercury was then dropped into the solvent where it
solidified instantly. After one-half hour the formation of a black
film of mercuric sulfide on the surface of the mercury was
observed. The film flaked into specks of black particles. This
example shows that the reaction of mercury with polysulfides is
rapid at low temperatures, and the mercuric sulfide produced is
easily removed from surfaces during the circulation of the solvent
mixture.
* * * * *